This invention relates to steam generators and more particularly to steam generators having modular tube bundles which can be removed and replaced to facilitate maintenance of the tubes in the steam generator.
Steam generators for pressurized water reactor power plants have been designed and constructed with a single large bundle of approximately 4,000 U-shaped tubes and arranged in such a manner that the outer periphery of the bundle is generally circular conforming to the shape of the cylindrical shell. The tubes are so disposed that one end of the tubes are located on one side of a tube sheet while the other end of the tubes are located on the other side of the tube sheet. A channel head forms headers through which primary fluid, hot pressurized water from the reactor, flows through the tubes to heat and boil secondary fluid or feedwater, which is supplied to the inner shell of the steam generator passing over the outside of the tube. Blowdown lines are provided to allow part of the secondary water to be drawn off to facilitate maintaining a balance chemical makeup and special materials are utilized in forming the tubes to provide a long life for the heat exchanger. However, leaks develop due to erosion or corrosion or a combination of the two. To provide separation of the primary secondary fluid the leaky tubes are plugged. While plugging a single tube results in only a small incremental and insignificance loss in performance of the steam generator, degradation of a large number of tubes from chemical action or mechanical wear by the support plates may eventually result in a significant loss performance. Therefore, replacement of large number of tubes would necessitate massive disassembly of the steam generator.